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An increase in roll size has meant six extra classrooms have been built at Whangarei Boys' High School, but the principal says the school has overestimated the growth and employed too many staff.

The new classrooms come as it is revealed that the value of consents for education buildings in Northland was up 30 per cent last year. Education buildings valued at $13.9 million were awarded consent last year in the region, an increase from $10.7 million in 2014.

A $15 million contract to rebuild Northland College - the biggest school building contract in the region for some years - has been awarded to Whangarei firm A Line Construction, but the consent will be issued this year.

Six pre-fabricated classrooms, which hold 23 students, have been placed on the field of Boys' High to cope with the school's growing roll which currently stands at 1155. The pre-fabricated classrooms are on lease from the Ministry of Education for two years.

However, principal Karen Gilbert-Smith said the school and Ministry of Education had over-estimated the roll growth, which had been expanding over time. In 2012, Whangarei Boys' High School has a roll of 1031 students, that increased to 1052 in 2013, 1101 in 2014 and 1155 in 2015. The Ministry of Education predicted the roll would reach 1277 by 2016, 1275 by 2017 and 1292 by 2018.

"It has not grown as much as we expected," she said "It's very difficult [to predict roll growth]. What we did was look at the last five years of retention percentage and we also look at contributing schools [like intermediate schools] and how many students might come to us from there, and base our predictions on that."

Ms Gilbert-Smith said transient students, students changing schools and a range of other factors meant sometimes that prediction was off, like it was this year. She said as a result the school had employed two too many staff.

"So we're about two people overstaffed and it is not ideal. If you ask teachers they will tell you [the current staffing] feels right."

The school gets Ministry of Education funding, based on the school roll size, for teachers' salaries.

"There is [also] board funded salary some schools use and we haven't used that and we prefer not to be in a position where we are forced to have that but the board have said they are open to maintaining current staff."

She said many Board of Trustees fund additional teachers from their own funds, if they have them, which may come from income generated through international students, for example. She said boards do this so they can offer a wider range of subjects or to have less students per class.

Ms Gilbert-Smith said despite the roll not reaching the number expected, the extra classrooms were needed with classes overflowing with students.

"What would happen is class sizes would increase and we would've just had insufficient space to manage so we needed these extra classrooms."

Ms Gilbert-Smith said there has been no date set for new classroom developments but said there was significant on-going work to be done between the Board of Trustees, the Ministry of Education and project managers Octa before anything else. She said she hoped there would be progress over the next year.